1. Field of the Invention
The present invention relates to a hydrophone group designed for use in shallow towed applications and particularly to a new mandrel for use in a hydrophone group designed for shallow towed applications.
2. Description of the Related Art
In general, a hydrophone is an acoustic pressure sensor designed for underwater use. A hydrophone detects acoustic wave signals under water. A hydrophone group generally comprises a plurality of hydrophone sensors, also called simply "hydrophones", towed behind a ship. In a hydrophone group, an optical source and photodetector are typically located on a towing vessel and connected through one or more optical fibers to a trailing network of hydrophones. One beam of coherent light is passed through one fiber in the hydrophones and subjected to underwater acoustic pressure, while another beam of coherent light is passed through a reference fiber not subjected to the underwater acoustic pressure and acts as a reference signal. The beams are brought to interference after transmission through an optical coupler, or the like. The optical fibers in the hydrophone groups undergo variations in refractive index and geometrical shape in response to local pressures under the water, i.e., in response to soundwaves. These variations cause phase modulation of the light beam passing through the hydrophone sensors. Soundwaves in the water are detected based upon the output signal. A hydrophone group requires a single interferometer. A typical hydrophone system uses a plurality of interferometers.
Towed hydrophone systems are typically used for geophysical exploration in which echo returns from active signals probing the ocean bottom are measured. Such arrays are towed in water depths of up to 50 feet and operate at acoustic frequencies of less than 100 Hz. The low frequencies allow an extended design of the individual hydrophones for flow noise averaging. In general, flow noise averaging is performed by summing the output signal of a hydrophone with the output signals of the other hydrophones in the group. Thus, a large number of hydrophones provide a good signal to noise ratio thereby helping eliminate the incoherent flow noise of the hydrophone system.
For shallow towed applications, a highly sensitive hydrophone system is useable. Previously, however, the sensitivity of the connection between adjacent individual hydrophones relative to the sensitivity of the individual hydrophone was a limiting factor in obtaining a high sensitivity by simply connecting several mandrels together. Specifically, the fiber extending between adjacent hydrophones, termed the interconnect, should be insensitive, providing a low responsivity to array bending and flexing.
The present invention sets forth a hydrophone group design which provides flow noise averaging and high sensitivity. Each hydrophone has a high sensitivity allowing discrimination against noise produced in the interconnects.